ASBMR: Space Travel Has Bone Risks for Men

Action Points

Note that this study was published as an abstract and presented at a conference. These data and conclusions should be considered to be preliminary until published in a peer-reviewed journal.

Explain that bone loss was greater for men than women astronauts after a stay on the International Space Station, and that a calculated risk of hip fracture was higher for male astronauts both before and after space travel.

Note that the number of female astronauts available for bone density studies was only five.

TORONTO -- Space travel may be one small step for women but one big fall for men.

Men who go into space lose more bone than female astronauts, and they also have an elevated risk of hip fracture when they return to Earth after a long time in orbit, according to Rachel Ellman, a PhD student at the Massachusetts Institute of Technology.

The finding adds another wrinkle to the understanding of the effects of spending lengthy periods in space, Ellman reported at the annual meeting of the American Society for Bone and Mineral Research here.

It was reported a decade ago, she noted, that astronauts lose bone rapidly in so-called microgravity -- 1.56% a month in the proximal femur, on average.

"This rate of bone loss," Ellman said, "is nearly 10 times that of post-menopausal women, who lose one to two percent a year." The loss occurs despite preventive measures such as exercise and vitamin D supplements, she said.

To investigate the issue, she and colleagues studied records of 25 astronauts, five of them women, who had spent about 170 days on average on the International Space Station. The astronauts had bone scans (of the whole body and the left hip) by dual energy x-ray absorptiometry before their flights, immediately after, and several times in the following years.

For the study, Ellman and colleagues also performed a so-called factor-of-risk analysis aimed at understanding what would happen if one of the astronauts had a sideways fall onto the left hip. The factor of risk was defined as the ratio of the force of the fall and bone strength.

The force of the fall, she noted, is a function of the astronaut's weight and height, as well as the thickness of the soft tissue overlying the hip. She and colleagues estimated bone strength by testing the breaking point of cadaveric femurs.

They found:

Weight and soft tissue thickness did not change markedly in flight.

During flight, men lost 6.7% of their bone mineral density, on average, compared with 3% for the women -- a difference that was significant at P=0.03.

The loss translated to a rate of 1.3 grams per cubic centimeter monthly for men, compared with 0.55 for women.

The factor of risk for hip fracturewas low for women -- about 0.4 on average, where 1.0 means a fall will lead to a fracture -- and did not change before and after a flight.

The average factor of risk for hip fracture was greater than 0.8 for men even before their flights, which was significantly higher than for women, at P<0.0001.

After their flights, the factor of risk was even higher for men -- 0.88 on average -- which was significantly higher than the pre-flight value, at P<0.01.

Ellman said that recovery of bone was incomplete and variable, and 78% of astronauts did not return to their baseline bone mineral density after 1.5 years.

She noted that the study had a sample size that -- while large for astronauts -- was small compared with most studies of the type.

But the study is a "little bit inconclusive," according to Tony Keaveny, PhD, of the University of California Berkeley, who was not part of the study but moderated the session at which it was presented.

"There was just a small number of women, so it's difficult to generalize," he told MedPage Today. The findings are "fascinating, but the challenge is that with astronauts you are working with tiny samples."

He noted that Ellman's data suggested that most astronauts never return to their baseline bone mineral density after a long flight. But that may be an illusion because of the natural loss of density that goes with aging, he said.

"We don't know where the astronauts should be going with their natural adult-related bone, so it's not clear if they got back to where they would have been," he said.

The study was supported by the National Institutes of Health. Ellman said she had no disclosures.

Accessibility Statement

At MedPage Today, we are committed to ensuring that individuals with disabilities can access all of the content offered by MedPage Today through our website and other properties. If you are having trouble accessing www.medpagetoday.com, MedPageToday's mobile apps, please email legal@ziffdavis.com for assistance. Please put "ADA Inquiry" in the subject line of your email.